[TowerTalk] Stainless Steel Galling

[Larry McDavid]

Stainless steel material inherently galls; it is a characteristic of the 
soft material. Most stainless steels have a softer surface than steel 
screws that rust; most stainless steels don't rust but are generally 
weaker than steel screws.

Passivation is a nitric acid process for stainless steels intended to 
remove any surface iron particles left from the alloy itself or from 
machining tooling; passivation is not a cleaning process and really does 
nothing to reduce galling. To test the effectiveness of the passivation 
process, sample parts are often tested for many hours in a salt spray 
(vapor actually) environment; if red or brown stains are seen after this 
test, the passivation process was not adequate. Successful passivation 
relies on first effectively cleaning the base stainless steel material, 
a fact often overlooked by the treatment facility!

Loctite makes several excellent anti-galling greases with copper or 
nickel particles that actually do the anti-galling work. No grease 
without these particles will substantially reduce stainless steel 
galling, though the grease will reduce friction initially.

Some of the common anti-galling materials discussed for amateur radio 
applications were actually developed for use on aluminum wiring 
terminations. The Loctite anti-galling materials should be superior for 
our mechanical fastener purposes. The Loctite anti-galling materials are 
available in small put-ups like grease sticks, about 1-inch diameter and 
four inches long. I put this material on every stainless steel fastener 
I use, for sure!

I've seen small stainless steel fasteners gall, especially if they were 
very clean before assembly, but you often see larger, structural 
stainless fasteners 1/4-inch and up gall if not protected by an 
anti-gall grease. If there is any microscopic motion in the joint due to 
varying stress loading (e.g. wind loading of structures), stainless will 
gall for sure! Stainless steels generally don't rust but they do have 
other problems.

My experience base? Forty years of precision-instrument mechanical 
engineering design.

Larry W6FUB


Grant Saviers wrote:
> Passivated SS, which is frequently the case for machine screw sizes (< 
> 1/4-20) is much less likely to gall.  Passivation tends to make the 
> screws more gray in color and less shiny.
> 
> Lanolin is also a "standard" anti gall lubricant for SS and is packaged 
> for this purpose and is a lot less messy than anti-seize compounds...
-- 
Best wishes,

Larry McDavid W6FUB
Anaheim, CA  (20 miles southeast of Los Angeles, near Disneyland)